COMP 401 INHERITANCE INHERITED VARIABLES AND CONSTRUCTORS. Instructor: Prasun Dewan

Transcription

1 COMP 401 INHERITANCE INHERITED VARIABLES AND CONSTRUCTORS Instructor: Prasun Dewan

2 PREREQUISITE Inheritance 2

3 MORE INHERITANCE Inheritance Graphics Examples Inherited Variables Constructors Memory Representation 3

4 POINT INTERFACE public interface Point { public int getx(); public int gety(); public double getangle(); public double getradius(); Read-only properties defining immutable object! 4

5 CLASS: ACARTESIANPOINT public class ACartesianPoint implements Point { int x, y; public ACartesianPoint(int thex, int they) { x = thex; y = they; public ACartesianPoint(double theradius, double theangle) { x = (int) (theradius*math.cos(theangle)); y = (int) (theradius*math.sin(theangle)); public int getx() { return x; public int gety() { return y; public double getangle() { return Math.atan2(y, x); public double getradius() { return Math.sqrt(x*x + y*y); 5

6 EXTENSION: MUTABLE POINT public interface MutablePoint extends Point { void setx(int newval); void sety(int newval); 6

7 CALLING CONSTRUCTOR IN SUPERCLASS public class AMutablePoint extends ACartesianPoint implements MutablePoint { public AMutablePoint(int thex, int they) { super(thex, they); public void setx(int newval) { x = newval; public void sety(int newval) { y = newval; Superclass constructor Inherited but not available to users of the class if not called from subclass 7

8 BOUNDED POINT public interface BoundedPoint extends MutablePoint { public MutablePoint getupperleftcorner(); public MutablePoint getlowerrightcorner(); public void setupperleftcorner(mutablepoint newval); public void setlowerrightcorner(mutablepoint newval); 8

9 ADDING VARIABLES public class ABoundedPoint extends AMutablePoint implements MutablePoint { MutablePoint upperleftcorner, lowerrightcorner; public ABoundedPoint(int initx, int inity, MutablePoint anupperleftcorner, MutablePoint alowerrightcorner) { super(initx, inity); upperleftcorner = anupperleftcorner; lowerrightcorner = alowerrightcorner; fixx(); fixy(); void fixx() { x = Math.max(x, upperleftcorner.getx()); x = Math.min(x, lowerrightcorner.getx()); void fixy() { y = Math.max(y, upperleftcorner.gety()); y = Math.min(y, lowerrightcorner.gety()); Variables also extended unlike in previous examples Superclass constructor 9

10 BOUNDED POINT IMPLEMENTATION public void setx(int newval) { super.setx(newval); fixx(); public void sety(int newval) { super.sety(newval); fixy(); public MutablePoint getupperleftcorner() { return upperleftcorner; public MutablePoint getlowerrightcorner() { return lowerrightcorner; public void setupperleftcorner(mutablepoint newval) { upperleftcorner = newval; fixx(); fixy(); public void setlowerrightcorner(mutablepoint newval) { lowerrightcorner = newval; fixx(); fixy(); 10

11 NEW CLASS HIERARCHY AND VARIABLES Object int x ACartesianPoint int y AMutablePoint MutablePoint upperleftcorrner ABoundedPoint MutablePoint lowerrightcorrner 11

12 CALLING CONSTRUCTOR IN SUPERCLASS public class ABoundedPoint extends AMutablePoint implements MutablePoint { MutablePoint upperleftcorner, lowerrightcorner; public ABoundedPoint(int initx, int inity, MutablePoint anupperleftcorner, MutablePoint alowerrightcorner) { super(initx, inity); upperleftcorner = anupperleftcorner; lowerrightcorner = alowerrightcorner; fixx(); fixy(); void fixx() { x = Math.max(x, upperleftcorner.getx()); x = Math.min(x, lowerrightcorner.getx()); void fixy() { y = Math.max(y, upperleftcorner.gety()); y = Math.min(y, lowerrightcorner.gety()); Super call 12

13 CALLING CONSTRUCTOR IN SUPERCLASS public class ABoundedPoint extends AMutablePoint implements MutablePoint { MutablePoint upperleftcorner, lowerrightcorner; public ABoundedPoint(int initx, int inity, MutablePoint anupperleftcorner, MutablePoint alowerrightcorner) { upperleftcorner = anupperleftcorner; lowerrightcorner = alowerrightcorner; fixx(); fixy(); super(initx, inity); Super call void fixx() { x = Math.max(x, upperleftcorner.getx()); x = Math.min(x, lowerrightcorner.getx()); void fixy() { y = Math.max(y, upperleftcorner.gety()); y = Math.min(y, lowerrightcorner.gety()); Subclass may want to override initialization in super class Super call must be first statement in constructor but not other methods. Superclass vars initialized before subclass vars, which can be used by thelatter Subclass vars not visible in superclass 13

14 CALLING CONSTRUCTOR IN SUPERCLASS public class ABoundedPoint extends AMutablePoint implements MutablePoint { MutablePoint upperleftcorner, lowerrightcorner; public ABoundedPoint(int initx, int inity, MutablePoint anupperleftcorner, MutablePoint alowerrightcorner) { upperleftcorner = anupperleftcorner; lowerrightcorner = alowerrightcorner; fixx(); fixy(); void fixx() { x = Math.max(x, upperleftcorner.getx()); x = Math.min(x, lowerrightcorner.getx()); void fixy() { Java complains that no super call y = Math.max(y, upperleftcorner.gety()); y = Math.min(y, lowerrightcorner.gety()); Wants Super class variables to be initialized No Super call 14

15 CALLING CONSTRUCTOR IN SUPERCLASS public class ABoundedPoint extends AMutablePoint implements MutablePoint { MutablePoint upperleftcorner, lowerrightcorner; public ABoundedPoint(int initx, int inity, MutablePoint anupperleftcorner, MutablePoint alowerrightcorner) { x = initx; y = inity; upperleftcorner = anupperleftcorner; lowerrightcorner = alowerrightcorner; fixx(); fixy(); void fixx() { x = Math.max(x, upperleftcorner.getx()); x = Math.min(x, lowerrightcorner.getx()); void fixy() { Manual initialization Java complains that no super call y = Math.max(y, upperleftcorner.gety()); y = Math.min(y, lowerrightcorner.gety()); Wants Object variables to be initialized 15

16 MISSING SUPER CALL public class AStringSet extends AStringDatabase { public AStringSet () { public void addelement(string element) { if (member(element)) return; super.addelement(element); No complaints 16

17 JAVA INSERTION public class AStringSet extends AStringDatabase { public AStringSet () { super(); public void addelement(string element) { if (member(element)) return; super.addelement(element); 17

18 MISSING CONSTRUCTOR AND SUPER CALL public class AStringSet extends AStringDatabase { public void addelement(string element) { if (member(element)) return; super.addelement(element); No complaints 18

19 JAVA INSERTION public class AStringSet extends AStringDatabase { public AStringSet () { super(); public void addelement(string element) { if (member(element)) return; super.addelement(element); 19

20 CALLING CONSTRUCTOR IN SUPERCLASS public class ABoundedPoint extends AMutablePoint implements MutablePoint { MutablePoint upperleftcorner, lowerrightcorner; public ABoundedPoint(int initx, int inity, MutablePoint anupperleftcorner, MutablePoint alowerrightcorner) { upperleftcorner = anupperleftcorner; lowerrightcorner = alowerrightcorner; fixx(); fixy(); void fixx() { x = Math.max(x, upperleftcorner.getx()); x = Math.min(x, lowerrightcorner.getx()); void fixy() { Could it insert the super call? y = Math.max(y, upperleftcorner.gety()); y = Math.min(y, lowerrightcorner.gety()); No Super call 20

21 CALLING CONSTRUCTOR IN SUPERCLASS public class ABoundedPoint extends AMutablePoint implements MutablePoint { MutablePoint upperleftcorner, lowerrightcorner; public ABoundedPoint(int initx, int inity, MutablePoint anupperleftcorner, MutablePoint alowerrightcorner) { super(initx, inity); upperleftcorner = anupperleftcorner; lowerrightcorner = alowerrightcorner; fixx(); fixy(); void fixx() { x = Math.max(x, upperleftcorner.getx()); x = Math.min(x, lowerrightcorner.getx()); void fixy() { y = Math.max(y, upperleftcorner.gety()); y = Math.min(y, lowerrightcorner.gety()); Does not know what the actual parameters should be Could initialized them to 0s and null values There may be multiple constructors, which should it choose? 21

22 MEMORY? Object int x ACartesianPoint int y AMutablePoint MutablePoint upperleftcorrner ABoundedPoint MutablePoint lowerrightcorrner 22

23 INHERITANCE AND MEMORY REPRESENTATION public class ACartesianPoint implements Point { int x, y; public class ABoundedPoint extends AMutablePoint implements BoundedPoint { MutablePoint upperleftcorner ; MutablePoint lowerrightcorner; ACartesianPoint@8 8 ACartesianPoint@16 ABoundedPoint@ new ABoundedPoint(75, 75, new AMutablePoint (50,50), new AMutablePoint (100,100)) 23

24 INHERITANCE AND MEMORY REPRESENTATION public class ACartesianPoint implements Point { int x, y; public class ABoundedPoint extends AMutablePoint implements BoundedPoint { int x, y; MutablePoint upperleftcorner ; MutablePoint lowerrightcorner; new ABoundedPoint(75, 75, new AMutablePoint (50,50), new AMutablePoint (100,100)) ACartesianPoint@8 8 ACartesianPoint@16 16 ABoundedPoint@48 Duplicate variables accessed 48 by subclass methods Inherited variables accessed by superclass methods Smalltalk did not allow redeclaration of superclass variables When making a super class out of subclass, accidental redeclaration can happen

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